Nowadays there are widely used contraceptive drugs based on the derivatives of estrogens and progesterones. These female steroid hormone combinations prevent pituitary gonadotropin release, thus preventing ovulation and subsequent fertilization. Pills of this type are taken 20 days per month and are quite safe and effective in preventing pregnancy. Sometimes undesired, and even quite serious side-effects are encountered. Recently also other drugs have been introduced for this purpose, such as a gonadotropin releasing hormone, which is effective in males and females by direct action on the gonads; prostaglandins which contract the uterine muscle and thus induce abortion; and antiprogesterone which competes with progesterone at its receptor and thus induces abortion. Each of the above contraceptive preparations has some limitations and sometimes rather severe side-effects or complications are encountered.